DE 102 04 939 A1 relates to a device for generating flat, parallel light. An optical unit, for use in registering the light that is reflected from a surface to be inspected, and an illumination system having at least two light sources, are provided. The optical unit and the illumination system are connected to a control unit. The light sources are arranged at a distance from one another. Each of the light sources emits light in a recording area of the optical unit, which recording area is provided on the surface to be inspected. The optical unit is oriented toward the surface to be inspected. At least one of the light sources of the illumination system, which light sources are arranged at a distance from one another, is subdivided into a plurality of individual light sources. The recording area of the optical unit lies within a plane of motion of the surface to be inspected, which surface is moved through this recording area. The spacing of the light sources of the illumination system extends in the direction of motion of the surface to be inspected. The individual light sources of at least one of these light sources are arranged transversely to the direction of motion of the surface to be inspected.
DE 10 2004 014 541 B3 discloses an optical system for use in generating an illuminated strip on a surface of a material. At least a portion of the surface of the material is reflective. Relative motion between the surface of the material and the illuminated strip is provided. An illumination device, having a plurality of light sources which are arranged side by side in a linear fashion and which are spaced at a distance from the surface of the material, and which emit light respectively at a spatial angle, emit light for generating the illuminated strip. A recording device records light that is reflected from the surface of the material with at least one detector which is positioned at a distance from the surface of the material. The illuminated strip is configured to have a width that extends orthogonally relative to its length on the surface of the material.
DE 199 30 688 A1 discloses a device for inspecting a surface. An inspection system, comprising an optical unit for registering the light which is reflected from the surface to be inspected, and an illumination system having at least two light sources, is provided. The optical unit and the illumination system are connected to a control unit. The light sources are arranged at a distance from one another. Each of the light sources emits light in a recording area for the optical unit, which recording area is provided on the surface to be inspected, with each light source emitting its light at a respective angle of incidence that is different from the others. The optical unit is oriented at a viewing angle toward the surface to be inspected.
DE 41 23 916 A1 discloses a device for use in the illumination-dynamic recognition and classification of surface characteristics of an object. At least one of the light sources of an illumination system, which light sources are arranged at a distance from one another, is sub-divided into a plurality of separate light sources. A control unit adjusts at least two of the light sources of the illumination system, which are arranged at a distance from one another, and/or the individual light sources of at least one of the light sources, which are arranged at a distance from one another, selectively and independently of one another.
The preferred use of the present invention is directed especially to the inspection of surfaces having non-uniform reflective areas. Surfaces of this type, with non-uniform reflective areas, are generally known, for example, in surfaces of a substrate material which is used for print products. Examples of this type of surface include products that are printed in security printing and in package printing processes, and especially in package printing processes for producing high-quality packaging, such as, for example, packaging for perfumes or for other luxury items. Security printing includes the production of, for example, bank notes, postage stamps or of documents, such as legal documents, share certificates, checks, and official identification papers or passes. Printed products having non-uniform reflective surfaces are typically produced, for example, in a special printing process. Such a special printing process can consist of a combination of known printing processes, such as of offset printing, intaglio printing or letterpress printing.
Non-uniform reflective surfaces are formed in these printed products, for example, by equipping the printing substrate with at least one special optical and/or haptic feature, in addition to the actual print motif. The haptic feature generally also has an optical effect. The special feature forms, for example, a security feature that serves to confirm the authenticity of the product and to make falsification of the printed product more difficult. However, the special feature may also be provided to form a particular aesthetic appeal for this printed product. For instance, in addition to the conventional ink that is applied, a special ink, such as, for example, an OVI ink, or optical variable ink, a lacquer, or a foil, can be applied to the surface of this printed product.
A haptic feature, which is one that is discernible by touch, is produced, for example, by providing the surface of the printed product with a relief-like structure. This may be accomplished, for example, through embossing and/or through a change in thickness of the printing substrate, or, for example, by giving the surface of the printed product a perforation, such as a micro-perforation. For example, a water mark, which is incorporated into the printing substrate, can be formed by varying the thickness of the substrate. With surface structures of this type, light that is incident at these points on the surface of the printed product is reflected differently from light that is incident on other areas of the surface of this printed product. Under certain circumstances, such incident light is not reflected at all.
An OVI, or optical variable ink, has the property that its perceived color changes depending upon its viewing angle, with such an ink changing color, for example, from reddish purple to olive green or to brown, due to the fact that incident light is refracted, scattered or reflected on pigments of the OVI ink. Special inks also may include fluorescent inks, for example, which contain pigments that luminesce, for example, in the dark or only with the incidence of light of a certain wavelength, such as a UV light. A similar optical effect may be produced, for example, by mixing mottling fibers into the printing substrate, which fibers luminesce in various colors under the incidence of, for example, UV light.
A lacquer, which may be applied especially to only a portion of the surface of the printed product, creates, for example, a glossy effect, which glossy effect alters the reflectivity of the surface of this printed product significantly as compared with an unlacquered area of the surface of the printed product. A lacquer, such as a transparent lacquer, can reflect incident light like a metallic surface. However, it will substantially decrease the visibility of a print motif disposed underneath the lacquer on the printing substrate, depending upon the angle of incidence of the incident light. A glossy strip, such as a shiny, gold-colored strip, which may be applied, for example, to a bank note as a security feature, also produces a particularly glossy effect.
At least one area on the surface of the printed product can also be configured with a material that is different from the printing substrate. The reflectivity of this different material may be different from that of the printing substrate. Areas of this type are formed, for example, by at least one foil element, such as, for example, by a metal, such as aluminum, or by a plastic, such as polypropylene or some other stable plastic. The foil element, in turn, can have a hologram. A security strip or a security thread can also be an element of this type. Sometimes, a foil element of this type also has a partial microstructure for the purpose of selectively altering its reflective property in a partial area. Obviously, therefore, foil elements can also give the surface of the printed product a non-uniform reflective structure.
Common to all of the embodiments of the special optical feature and/or the special haptic feature mentioned above, by way of example, is that they present a special challenge when one or even when several of these different features are to be inspected at the same time as is the print motif applied to the substrate, with such an inspection being done in order to assess the quality of the printed product. This is because the substrate itself, and the conventional ink printed onto that substrate, to produce the print motif, generally reflect incident light diffusely in a larger solid angle, in other words scattering it, without a pronounced preferred direction. This is typically due to their irregular and therefore rougher surface quality. A metallic or metalized foil element, which may be incorporated into the printing substrate or applied to its surface, or a lacquer that is applied to the surface of the printing substrate, generally reflects incident light in a preferred direction in accordance with the law of reflection, due to its regular and therefore smoother surface property. According to the law of reflection, the reflection angle corresponds to the angle of the incident light, and these two angles, along with the perpendicular of incidence, which is based upon the surface of the printing substrate, lie within the same plane.
An inspection system for use in inspecting the quality of a printed product has, for example, an optical unit which is used for registering the light that has been reflected from the surface to be inspected. This optical unit is preferably an imaging unit, such as, for example, a camera system, and especially a camera system for use in recording a color image. The inspection system also includes an illumination system for illuminating the surface of the printing substrate, or at least for illuminating the non-uniform reflective areas of the surface of this printing substrate. An inspection system of this type is described, for example, in the prior art documents DE 198 59 512A1 or DE 100 61 070A1.
The above-discussed challenge of inspecting a printed product having non-uniform reflective areas on its surface arises because the ability to inspect non-uniform reflective areas is optimal only when an angular position between the camera system and the illumination system is adjusted based upon the reflectivity of the respective surface.